Nanoparticles have been explored as fillers in polymers to improve various properties such as mechanical, thermal, electrical, and barrier properties. The driving force for the use of nanofillers is the enormous specific surface area that can be achieved as the size of the fillers reduces to less than 100 nm. Nanoparticles possess orders of magnitude higher specific surface area than their micron and macro sized counterparts. This can lead to two relevant phenomena. First, there is an increased area of interaction between the filler and the matrix. Secondly, there is a region surrounding each particle in which the polymer behaves differently from the bulk. The volume fraction of this “interaction zone (IZ)” can be larger than the volume fraction of particles and the properties of the IZ contribute to the change in properties. The increased interaction can have a variety of effects. It can lead to a change in the glass transition temperature (Tg) and the load transfer from the polymer to the fillers. In the case of a semicrystalline polymer, the increased interaction can result in a change in the crystallization behavior such as the crystallization temperature (Tc).
However, the extent of these changes depends on the interface between the fillers and the polymer. In general, a stronger interface results in better load transfer and a higher Tg. The route to achieving a strong interface is to alter the surface of the nanoparticles by coating it with molecules that are either compatible or can bond to the polymer molecules. Most attempts to alter the surface of nanoparticles involve coating the nanoparticle surface with coupling agents such as silanes or phosphonic acids that have one end group that is adsorbed on the particle surface and the other end group that is compatible with the polymer molecule. Then bonding is achieved by grafting a polymer on a filler that involves reacting the coupling agent with a monomer molecule followed by polymerization. While this method has shown promising results, problems associated with the foregoing include difficulty in controlling the molecular weight and molecular weight distribution.
Alternative methods have been used to increase the glass transition temperature of a polymer, such as a polyester, to improve its thermal stability. One method is to blend the polyester with another polyester having a higher Tg. For instance, PET (Tg˜80° C.) has been blended with polyethylene naphthalate (PEN), which has a Tg of 130° C. A disadvantage of this process is the high cost of polyesters with higher Tg. Co-polymerization is another route employed for increasing Tg of polyesters. Copolymers of PET with PEN have been prepared and the Tg increases with increasing naphthalene units. PET has also been co-polymerized with 5-nitroisophthalic units (PETNI) to obtain increases in glass transition by up to 6° C. for 50% NI content. But this method requires a high percentage of comonomer units to achieve appreciable changes.
A need exists for polymers filled with nanoparticles, i.e., polymer nanocomposites, and methods of preparation thereof that overcome at least one of the aforementioned deficiencies.